Bleaching of pulp for papermaking is attained in multistage bleaching treatment. Heretofore, in the multistage bleaching, a chlorine-base bleaching chemical is used as the bleaching agent. Concretely, by a combination of chlorine (C), hypochlorite (H) and chlorine dioxide (D), bleaching is attained in a sequence of, for example, C-E-H-D or C/D-E-H-E-D (C/D means a chlorine/chlorine dioxide combined bleaching stage; and E means an alkali extraction stage).
However, in bleaching with them, these chlorine-base bleaching chemicals release, as side products, organic chlorine compounds that are harmful to the environment, and the environmental pollution with the bleaching waste that contain those organic chlorine compounds is considered problematic. Organic chlorine compounds are analyzed and assessed generally by an AOX method, for example, by the US Environment Agency (EPA: METHOD-9020).
For reducing and preventing release of organic chlorine compounds as side products, it is most effective to reduce the amount to be used of chlorine-base chemicals or not to use them; and in particular, it is a most effective method not to use elementary chloride in the initial stage. Pulp produced according to a method of not using elementary chlorine but using chlorine dioxide is referred to as ECF (elementary chlorine-free) pulp; and pulp produced according to a method of using no chlorine-base chemical at all is referred to as TCF (totally chlorine-free) pulp.
As a method of bleaching cooked/oxygen-deligninated pulp with chlorine dioxide, not using elementary chlorine in the initial stage, generally known is a sequence of D-Eo-D, D-Eop-D, D-Eo-D-D, D-Eop-D-D, or a sequence of D-Eo-P-D or D-Eop-P-D (p or P means hydrogen peroxide, Eo means an oxygen alkali extraction stage, Eop means an oxygen/hydrogen peroxide alkali extraction stage); and as a bleaching method of using ozone (Z) in the initial stage, generally known is a sequence of Z-Eop-D, Z-Eo-P-D or Z/D-Eop-D (″/″ between Z and D means continuous treatment with no washing therebetween).
However, chlorine dioxide and ozone are inferior to chlorine conventionally used in point of the ability to remove hexeneuronic acid (hereinafter abbreviated as “HexA”), and therefore a large amount of HexA remains in the bleached pulp. The remaining HexA is a cause of the deterioration of the colour reversion resistance of ECF or TCF-bleached pulp.
HexA is a substance resulting from demethanolation in a cooking step of glucuronic acid bonding to xylan, a hemicellulose existing in pulp. Though having a small influence on the brightness of pulp, this reacts with potassium permanganate, as having a double bond in the molecule, and is counted as a potassium permanganate (K) value or a κ (kappa) value.
As a papermaking method, there are known an acid papermaking method of using aluminium sulfate, and a neutral papermaking method of using calcium carbonate. Neutral paper worsens in point of the colour reversion resistance with the increase in the HexA content thereof, but the degree of worsening is low; and the colour reversion resistance of acid paper made by the use of aluminium sulfate particularly worsens. The reason why the colour reversion resistance of paper made according to an acid papermaking method is unknown at present, but the existence of HexA and the use of aluminium sulfate may be the reason for it.
In a paper mill, in general, neutral paper and acid paper are made separately from chlorine-free bleached pulp forwarded from a series of bleaching equipment, by the use of a large number of papermaking machines. Accordingly, using the same chlorine-free bleached pulp forwarded from the same bleaching step, acid paper is made in one line and neutral paper is made in the other line. In this case, though the paper made in the neutral papermaking line has no problem, the paper made in the acid papermaking line may have a problem of the colour reversion.
For preventing the colour reversion resistance deterioration, it is necessary to increase the amount to be used of chlorine dioxide or ozone having the ability to remove HexA, thereby removing HexA. In this case, however, the pulp for neutral paper not requiring measures against the colour reversion must also be bleached, therefore increasing too much the brightness of the pulp and causing a problem of great increase in the cost for bleaching.
In place of delignination of unbleached pulp by chlorine bleaching or by a combination of chlorine and chlorine dioxide, known is a method of applying a peracid such as peroxomonosulfuric acid to bleaching (for example, see Patent References 1 to 7). Patent Reference 1 proposes a TCF bleaching method comprising treatment with peroxomonosulfuric acid and then with alkaline hydrogen peroxide.
Patent Reference 2 proposes a bleaching method by a combination of enzyme and peroxomonosulfuric acid.
Patent Reference 3 proposes a bleaching method by chelating agent treatment, alkaline hydrogen peroxide treatment and peroxomonosulfuric acid treatment after oxygen bleaching.
Patent Reference 4 proposes a bleaching method by a combination of peroxomonosulfuric acid and ozone.
Patent Reference 5 proposes a method comprising peroxomonosulfuric acid treatment after chelating agent treatment, and then alkaline hydrogen peroxide treatment in that order.
The methods disclosed by the above-mentioned Patent References 1 to 5 are methods relating to initial-stage delignination treatment in a bleaching step, but the references have no description relating to HexA removal and the colour reversion resistance enhancement.
Patent Reference 6 proposes treatment with a peracid and an alkaline earth metal in the final stage of bleaching. As the peracid, used is peracetic acid; but the main object of this method is for brightness improvement; and the reference has no description relating to HexA removal and the colour reversion resistance enhancement.
Patent Reference 7 proposes a method of adding a bleaching agent between bleaching treatment and a preparation step, as a post-treatment method after bleaching. As a bleaching agent, shown are ozone, hydrogen peroxide, peracetic acid, percarbonic acid, perboric acid and thiourea dioxide; but the main object of this method is for brightness improvement, and the reference has no description relating to HexA removal and the colour reversion resistance enhancement.
[Patent Reference 1] JP-T 6-505063
[Patent Reference 2] JP-A 7-150493
[Patent Reference 3] JP-T 8-507332
[Patent Reference 4] JP-T 8-511308
[Patent Reference 5] JP-T 10-500178
[Patent Reference 6] JP-T 2001-527168
[Patent Reference 7] JP-A 2004-169194